Since a lithium-ion secondary battery has a high energy density, it has widely utilized as power source for small-sized electronic equipment such as a portable phone or a notebook computer. It has been recently desired to allow the battery to realize a larger capacity and a higher power output in order to apply it to a power source for electric auto vehicle. Although LiCoO2 is mainly used as positive electrode materials for a currently used lithium-ion secondary battery, it has problems such that cobalt (Co) is extremely expensive and has a very high toxicity. Therefore, a new material has been recently developed to aim at a positive electrode using no cobalt.
In the development of the new positive electrode material, an attention is riveted to any olivine type compound LiMPO4 (M is Fe or Mn) composed of Fe and/or Mn which is (are) electrochemically stable and resources of which are rich. It, however, has an electronic conductivity of about 10-9 Scm−1, which is low, and has also a diffusion velocity of Li ion in a crystal of 10−14 through 10-16 cm2S−1, which is excessively late, unless it is treated. Thus, it has such an excessively poor rate performance and a problem so that it is difficult to obtain a sufficient output or capacity.
Regarding LiFePO4, recently, it has been clearly shown that its performance can be improved by means of nano-sizing method, carbon composition method or the like, which causes its application to be desired. Further, a patent document 1 discloses positive electrode materials in which in olivine type LiFePO4, its conductivity is improved by changing a part of Fe to Ti and which realizes larger capacity and higher rate performance by processing them as fine particles.
On the other hand, LiMnPO4 has a discharge potential of 4V (vs. Li/Li+) which is desired to have higher energy density and higher output than those of LiFeO4 that has a discharge potential of 3.4V (vs. Li/Li+) . Since LiMnPO4, however, has been inferior to LiFeO4 in electronic conductivity or diffusion of Li ion, its sufficient performance has not yet shown under the existing circumstances.